1. Field of the Invention
The present invention relates to a method for forming a solder bump used to mount, for example, an IC chip on, for example, a printed circuit board, an apparatus for forming the solder bump, a head unit for use therein and a soldering method using the head unit.
2. Description of the Prior Art
When mounting, for example, an IC chip on a printed circuit board, it is necessary to form a solder bump for causing an electrical contact between the IC chip and the printed circuit board and fixing the IC chip onto the printed circuit board. In recent years, the mounting density on the substrate has increased, the number of the solder bumps has increased, and the disposition pitch between the solder bumps has become narrowed.
Conventionally, as a method for forming a solder bump, for example, solder paste is printed on a board having copper pads formed thereon beforehand and the resulting board is heated and reacted within a reflow furnace. Thereafter, the solder paste on other portions than the copper pads is removed by cleaning, whereby the solder bumps are collectively formed on a large number of the copper pads.
As another method for forming a solder bump, using a heater contained in a solder discharging device made up into a syringe structure, the solder in the cylinder is fused and, while pressure control is carried out by a piston, the thus-fused solder is discharged in a very small amount from a fine opening and adhered onto a copper pad.
As still another method for forming a solder bump, it is known to set a thin film-like solder sheet between a punch of a fine cross section heated by a first heater and a die heated by a second heater, lower the punch and soften the solder sheet to thereby punch it out, and adhere the punched-out solder sheet onto a copper pad using an adhesive flux coated on the copper pad in advance.
However, the conventional solder bump forming methods have the following problems:
1) In the first method, there are cases where it is difficult to adhere a required amount of solder onto the substrate through the execution of only a one-cycle process alone. In these cases, since several cycles of adhering process are repeatedly executed, the productivity becomes deteriorated. PA1 2) In the second method, when the discharged fused solder is adhered onto the copper pad, the fused solder is separated within the cylinder and in addition the separated positions vary. Therefore, it is difficult to supply a minute and fixed amount of solder stably. PA1 3) In the third method, although the punch of a very small cross section is used, heating a forward end of such punch by thermal transfer from an external heater is inferior in terms of the heat efficiency. For this reason, in the third method, a large-sized heating mechanism must be provided in the vicinity of the punch, with the result that the resulting apparatus becomes one which is difficult to move. Also, there is the problem that heating, softening and punching out the solder by means of the punch are not easy. PA1 1) First, the solder in an amount to be adhered onto the substrate is adhered onto the end surface of the light waveguide member on a side opposite to that on which the light heating source is coupled thereto, by the use of, for example, the adhesiveness. PA1 2) Subsequently, when a heating light is output from the light heating source coupled to the light waveguide member, the heating light passes through the light waveguide member and is emitted from the end surface thereof having the solder adhered thereonto and is radiated onto the solder adhered thereon. By adjusting the energy of the radiated heating light to an intensity with which the solder is fused, the solder adhered on the end surface of the light waveguide member is fused and shaped like water drops. PA1 3) Subsequently, when the water drop-like solder adhered on the end surface of the light waveguide member is brought into contact with a desired position on the substrate (e.g., the pad portion or the solder bump portion at which the amount of solder adhered is wanted to be increased) by using the guiding mechanism, the fused solder is adhered onto the pad portion side or the solder bump portion side at which the amount of solder adhered is wanted to be increased, having a higher wettability than the end surface of the light waveguide member. Thus, the solder bump is formed.
Also, in a case where in this method several solder bumps are defective (the amount of the solder is smaller than a proper amount of solder), when the disposition pitch of the bump is narrow, it is difficult to add a solder paste to only each of these defective solder bumps and print it to thereby increase the amount of the solder adhered and thereby correct it to a proper amount of solder. It is to be noted that although in this case it is possible to prepare a mask for adding a solder paste and printing the same and thereby perform additional printing thereof, this causes an excessive increase in the cost.
Also, the copper pad onto which solder is to be adhered, or defective solder bumps which already exist, have their surfaces oxidized and so there are many cases where the fused solder is difficult to adhere onto these surfaces. Accordingly, when adhering solder, there is the troublesomeness of separately coating flux as a reducing agent.
Further, when solder, for example, solder paste is adhered onto the copper pad on the substrate, unless solder paste is reliably adhered onto the copper pad, an excellent solder bump cannot be formed. Also, when solder paste is adhered onto the copper pad, there is also the problem that squashing this solder paste excessively by means of a forward end of a member retaining the solder paste causes the solder paste to be severed whereby the formation of an excellent solder bump is obstructed.